I am no expert, but I think you're on the right track. I love the way the grass on the dune looks.
I also think you're right on about the weak points; shading on the water would be really helpful (right now, it looks like a flat plane; adding some shading could give it some depth), giving the wind more motion would make it much more evocative, and the trees look pretty sparse; an impression that could be helped greatly by making them look more impressionistic (the style you're talking about is called "pointillism," by the way).
Also, I love the texture and color you gave the sky, but in my experience the sky by the sea tends to gave a gradient of brightness, depending on where the sun is. Same with a dune. Giving both some more shading would add a lot of atmosphere.
Good work, and good luck!